Method for developing electrostatic images



March \5, 1957 L. E. WALKUP 2,784,109

METHOD FOR DEVELOPING ELECTROSTATIC IMAGES Filed Sept. 18, 1950 3 Sheets-Sheet l s: 5. Fl 1 3 4 2o INVENTOR.

LE WIS E. WALKUF MW (3942:...

ATTORNEYS March 5, 1957 L. E. WALKUP METHOD FOR DEVELOPING ELECTROSTATIC IMAGES 5 Sheets-Sheet 2 Filed Sept. 18. 1950 Fig. 4

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METHOD FOR DEVELOPING ELECTROSTATIC IMAGES Filed Sept. 18, 1950 s Sheets-Sheet 3 FROM COMPRESSED AIR SUPPLY INVENTOR. v LE Ms E. WALKUP u WGMA lusuuvnou W s9 3 A'r'rolalveiYs United tates Patent NIETHQD FOR DEVELOFING ELECTROSTATIC IMAGES Lewis E. Walitup, Columbus, Ohio, assignor, by mesne assignments, to The Haloid Company, Rochester, N. Y., a corporation of New York Application September 18, 1950, Serial No. 185,387 9 Claims. (Ci. 117-175) This invention relates to the development of electrostatic or electrophotographic images and to methods and apparatus for carrying out such development.

The invention has application to the development of electrostatic latent images on electrophotographic plates, such as those described in U. S. Patent No. 2,297,691 to Chester F. Carlson, for example, and other electrostatic images on insulating surfaces. In the process of electrophotography, as described in the Carlson patent, an electrophotographic plate is used comprising a layer of photoconductive insulating material which is usually atfixed to a conductive backing plate or film. In the usual method of carrying out the process the plate is first given a uniform electrostatic charge over its surface. The photoconductive layer is then exposed to a light pattern or image in a camera or by contact exposure methods, for example, to bring about a discharge or partial discharge of the electrostatic charge in the areas where light strikes the layer and leave an electrostatic latent image on the layer. If exposure has been made correctly the areas which received the least light retain the highest charge while those which received the most light retain the least charge. Areas which received an intermediate degree of illumination retain an intermediate charge.

The latent image can then be developed by depositing a finely-divided electrostatically attractable material, such as a powder or fine liquid droplets onto the surface of the layer where it is trapped by the electrostatic charges of the image. Where the charge contrast is greatest the greatest amount of material is deposited and where the contrast is least little or no material is deposited. It is thus possible to produce a powder image, for example, substantially conforming to the light image originally projected onto the layer. The powder, or:other material, can subsequently be transferred to a sheet of paper or other surface and fixed to produce a permanent print.

Images of line material (drawings, typewriting, printing and the like) and half-tones are reproduced with good fidelity by the electrophotographic process. Continuous-tone images, namely images in which the density varies through several shades of gray as in photographs, radiographs and the like, tend to be reproduced as somewhat contrasty prints by electrophotography. That is, intermediate shades of gray are not all reproduced proportionally. It has also been found that large black or dark areas do not always develop uniformly throughout their whole area but tend to develop more heavily around trodes spaced from the image surface and in methods and means for applying control potentials to said electrodes thereby to control the deposition of a developing material, such as a finely-divided electrostaticallyattractable material, on the electrostatic image in a predetermined manner.

The invention comprises the features of construction, combination of elements, arrangement of parts, and methads of operation referred to above or which will be brought out and exemplified in the disclosure hereinafter set forth, including the illustrations in the drawing.

In the drawings:

Figure 1 is a top view with parts broken away, of a developing device embodying features of the present invention, said device being arranged for development of.

art and according to aspects of the present invention, respectively;

Figures 5, 6, 7 and 8 are graphs illustrating certain characteristics of the exposure and development processes for electrophotographic plates;

Figure 9 is a circuit diagram showing an electric circuit comprising part of the apparatus arranged according to a modification of the invention;

Figure 10 illustrates a modified arrangement suitable for development of an electrostatic image according to a modified technique embodying the use of a sprayed cloud of liquid mist;

Figure 11 is a detail of the liquid spray device; and

Figure 12 illustrates a powder spray device.

, While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the construction and arrangement of parts and in the method of operation without departing from the spirit of the invention. In the following description and in the claims parts will be identified by specific names for convenience, but they are intended to tator brush 22 driven by electric motor 23. The top' fianged'end of the cone is clamped beneath a top plate assembly comprising plates 24 and 25 provided with a central rectangular aperture over which a plate 26, such as an electrophotographic plate, carrying an electrostatic latent image, is placed face downward for development. A development control electrode 27 comprising an array of conductors, such as a fine wire grid is mounted in the aperture in spaced relation to the face of plate 26 and connected to a conductor 28 for the application of suit able control potentials to grid 27.

The device is mounted on a flat base or bedplate 29 by four vertical legs 3t which support a rectangular frame 31. The top of cone 21 is provided with a horizontal flange 32 extending outwardly and resting on frame 31. Plate 24, formed of insulating material such as phenol-formaldehyde resin, plate glass, dry wood or other suitably rigid insulating material, is secured on top of flange 32 with its central rectangular aperture 33 disposed directly over the center of the cone. Sheet 25, which is afiixed to the top surface of plate 24, is of thin stock, preferably 0.01 inch or less in thickness, and is '3 also preferably formed of rigid insulating material such as phenol-formaldehyde resin, polystyrene or the like. The central aperture 34 of this sheet is also rectangular in shape but is slightly smaller than aperture 33 so as to provide an overlapping flange around the edge of the aperture 33, as illustrated.

Plate 26, carrying the electrostatic latent image to be developed, may comprise an electrophotographic plat: formed of a flat metal plate having a coating 35 ofpll'o'toconductive insulating material, as described for example in Carlson Patent No. 2,297,691. For purposes of development the plate is placed face down over aperture 34 with coating 35 substantially in the plane of the upper surface "of sheet 25.

Developrnentcontrol grid 27 issupported directly underneath the face of coating 35 in spaced parallel relation to it. Gri'd27 may comprise a series or fine, parallel wires stretched taut between the two sides of a 'rigid'reetangniar metal frame 3 6'. Th wire can be 'strung'baei; and forth over pins 127 on the outside edges of frame 36, Frame 36; is larger than aperture 34 but slightly Smaller than aperture 33 so as to iit up under the fiang'e provided. by sheet 25 where it overhangs the aperture 33. This enables grid 27 to be brought close to the surface of coating 35 without danger of direct contact. Generally speaking, relatively close spacings are preferred. Frame 36 is provided witha pairof outwardly extending lugs or flanges 37 spaced from the lower face of plate 24 by spacing washers 38 and secured to the plate by screws '39 to .provide accurate location of the grid 27. In a preferred embodinient, grid 27 may be formed of 0.0035 inch diameter stainless steel wire and be mounted so as to provide sixty equidistant parallel wires to the inch. While grid to plate spacings between grid 27 and coating 35 of 0.010 inch to 0.5 inch are useful, the most satisfactory range for the grid described is between 0.040 and 0.080 inch.

Gone 21 ispreferably formed of metal, such as brass, aluminum, steel or the like. The angle of flare-is not critical but for practical operation it is desirable that the conical wall have a sufliciently steep slope to allow-develo'pingpowder used in the chamber to slide to the bottom either without agitation, or with a slighttappingor vibra tion of the chamber. -The inside surface is preferably smooth or polished to facilitate sliding of the developing powder. The. bottom of the chamber is closed by a horizontal, circulardisk-shaped wall 40 having a-central apertur'e accommodating the upward extension of rotary shaft 41 upon which brush 22 is mounted within; the chamber. By way of, example, disk-shaped area-40 may have! diameter t '2." and brush 22 may comprise a .pair of oppositely extending one-inch tufts of bristlcs-iQrmcd-Qf horsehair or other rather stifi bristles. Shaft '41 issupported'vertically for rotation in ball. bfijal'ingbSSfifllblii 42'a'nd' 'li and assembly 42 being set into baseplate 29 and assembly '43 being supported by a bracket. just beneath the bottom "of chamber 21. The ten wom 14 surrounding the shaft seals the central aperture inbottom disk '40 against powder leakage. Shaft 41 isv provided with a pulley 46 and electric motor 23 mounted with its shaftvertic'a'l on fra'me-crosspiece 47 is 'provide'dwit'h a pulley-48. A V-belt 49 threaded over pulleys 46 and4'8 provides a driving connection-between the motonand'sha'it 41. With an 8" high chamber 21 "having a 2 diameter brush '22 at the bottom, a brush speed of 2500 "P M. affords suitableagitation for 'powderplacedin chamber 21 in order to develop a'cloud of powder within the chamber. A 1725 R. P. M. electric motor may-beiused with pulley sizes adjusted. to provide the desired bru'sh speed.

A potential source such as 'a-battery smwhiehmay have a-potential of scveralhundred volts is 'provide'diior applying a potential difference between. grid 27' and backs in g plate26. whendesired. In order to varythelpot'entialapotentiometersl is connected between the terminals of battery 50 and sliding contact 52 of the potentiometer posed predominately is connected by conductor 28 to grid frame 36 and, hence, to grid 27 A spring contact 53 mounted on an insulating support post 54 on top of plate 25 is arranged to rest against the back of plate 26 to connect it to ground. A center tap of potentiometer 51 is also grounded. It is thereby possible, by moving sliding contact 52 to adjust the potential of grid. 27 to be equal to the potential of plate 26, i. e. at ground potential, or to make grid 27 up to several hundred volts positive or negative with respect to plate 26.

Preparatory to developing an electrostatic latent image with the device illustrated in Figures 1 and 2, a charge of developing powder 55 is placed in conical chamber 21. Suitable powders for this purpose comprise finely-divided black or colored powders either alone or mixed with a granular material having difierent triboelectric properties in order to generate electrostatic charges on. the individual powder particles. By way of example. one suitable develop'er consists of the following composition.

EXAMPLE 20cc. of a finely-divided developer powder and 5, .cc. of a granular carrier material mixed together. A suitable developer powder may be produced by blending together 20 parts of a rosin-modified phenolformaldehyde resin having amelting point in the orderof 142-149" C., such as is sold commercially as Amberol F-7l Resin by The Resinous Products Division of Rohm and Haas Company, Philadelphia, Pennsylvania, and 1 part of carbon black, such as that sold commercially as'Raven Bead Carbon Black. The blendis fused and thoroughly mixed, allowed tosolidify, crushed and micronized to provide a particle sizein which mostof the developer particles haven-diameter in-thc range between 1 and 20 microns. The carrier may consist of soybean protein from which the oil has been removed, cut to a size which will-pass, through a 16'- mjesh screen :and be retained on a 48-mesh screen.

Qther developingpowders and developing mixtures may also be used-in the developing device. Where a carrierpowdermixture is used, the powder and carrier are:prefer ably formed of materials so situated in the triboelectric series that charges are imparted to the powdcr particles of a polarity opposite to the charge on the electrostatic latent :imagcnto be developed. Thus, if the latent image is-made upofipositive electrostaticcharges, itis preferred that-the fiuely divided. developer be formed of acompositicw hich will acquire negative charge by contact with tlicfcarricr-material. In other words, the developer .is formed of amaterial'which is nearer to the:negative=:end ofthetrihoelectricxseries than the carrier material.

The -result' of a agitating such a powder carrier mixture ismnmate a2powder cloud in chamber .21 which; is :com- 2 of negatively charged powder :particles. The carrier particles, being much larger and heav ien tend to fall back to-zthe bottom of the chamber-sand do'notrise :to the developing area at i thetop.

:In operation the developing device may be used t9 develop :an electrostatic latent .imageon an electrophoto' graphic plate-26in, theYfolloWing manner. The latent image on coatingiSS: of-the electrophotographic plate: may be prdducedinany'known'manner. For example,:a.posi tively charged image may :be formed by first placing--21 uniform. positive electrostatic:charge -over the surface 05 coating :35: byfsubjectingi it tofa' positive electric: discharge from a corona discharge J electrode and then exposing the coating by contact'iexposure'with an original to be copied or try-exposing the plate in a camera to any subject or acene to bezrecorded. This exposure discharges" the'pl-ate iu' picportion to theslightrreceived'leaving the electrostatic latent imageion the. coating. The plate is then placed'face down over aperture .34 as illustrated in Figure 2 and iground contact sii' allowed'toi rest onthe back of the plate.

Tap 521':is'iadj-usted tothe desired potential, as will" be described? more in detail later, and then, with aeharge SS' of developer and carrier mixture in the chamber 21, electric motor 23 is started to rotate brush 22 at a speed such as. 2500 R. P. M. This produces active agitation of developer mixture 55 and causes a cloud of predominantly negatively charged powder particles to rise in chamber 21, some of the particles rising through grid 27 into the space between the grid and coating 35 where they are attracted to the positively-charged areas of the image and form a powder deposit on coating 35 corresponding to the electrostatic latent image.

With the chamber 8" high, an operation of the agitator brush 22 for between one-half and three minutes is usually sufiicient to product adequate development of the electrostatic latent image. If desired, tap 52 may be moved during development to vary the potential on grid 27 in relation to plate 26 over a range of values which may include potentials both positive and negative with respect to the plate backing as well as equal to the plate potential as will be described more in detail later. If excessive quantities of developer accumulate on the sloping walls of chamber 21, it will be found desirable to tap the chamber occasionally to shake the loose powder down to the bottom. Occasional cleaning of grid 27 may also be required but this needs usually be done only once prior to placing the plate on the apparatus.

After development plate 26 may be removed from the device and the powder image transferred to a sheet of paper or other base by any suitable method such as by pressure contact, or by passing the plate with a sheet of paper laid loosely over the developed coating under a positive corona discharge to attract the negatively-charged powder image to the paper. After transfer the powder image may be fused to the sheet to produce a permanent print. Any loose residual powder on coating 35 may then be removed and the plate used again to form another electrostatic latent image which can be developed in the same manner.

The function of development control grid 27 in controlling the development of the latent image may be more fully explained by reference to Figures 3 to 8. Figure 3 illustrates the effect of developing an electrostatic latent image on an electrophotographic plate with a powder cloud without the use of a development control electrode. The group of plus signs 57 on coating 35 indicate a charged area of the electrostatic latent image corresponding to a dark area of the original image which was used in making the exposure. It would obviously be desirable to develop this area of the coatingheavily with powder in order that the resulting picture would have a dark area corresponding to the dark area of the original. However,

it will be noted that coating 35 is alfixed to conductive Y base plate 26 and hence the positive charges on the coating induce corresponding negative charges underneath the coating in plate 26 so that coating 35 acts very much like the dielectric of an electrostatic condenser. The net effect on negatively charged powder particles 56 floating in the air near the surface of coating 35 is the sum of the effects of positive charges 57 on the coating and negative charges 58 induced in backing plate 26. Since coating 35 is very thin and charges 58 are almost equal in amount to charges 57, the net effect on particles 56 is only very slight. Another way of expressing it is to say that very few lines of force coming from the positive charges extend out into the air where the powder particles are floating. Most of the lines of force pass directly through the coating 35 to the backing plate and hence are not available to influence the powder particles to deposit on the coating. Near the edges of the area 57 of positive charges a few lines of force will pass through the air in curved paths which terminate in negative charges in base plate 26 around the margin of the charged area, as shown in Figure 3. These lines of force will capture a few negative powder particles 56 along the edges of area 57 resulting in the development of a rather heavy border around area 57 while leaving a very light, practically undeveloped Central area. It will be apparent from this that the tones of the original being copied will not be very faithfully reproduced in the powder image. Where small dark areas are involved, such as lines, letters, characters and the like, good development is obtained but large dark areas are not developed uniformly and continuous-tone images tend to be contrasty.

Figure 4 illustrates the improvement obtained by adding control grid 27. Grid 27 provides a conductor spaced in front of coating 35 thereby producing'a capacitance between charged coating 35 and grid 27. This tends to intensify the number of lines of force extending outward from the charges on coating 35 and reduce the number extending through layer 35 to the conductive backing. The grid also tends to straighten out the lines of force at the edges of charged area as can be seen in Figure 4. The result of this change is to intensify the field in the air near the surface of coating 35 so that negatively charged powder particles 56 floating in this region will be more strongly attracted to all parts of charged area 57. Moreover, due to the straightening out of the lines of force at the edges of the charged area, the tendency to develop a heavy border around the charged area is greatly reduced or substantially eliminated. The result is the uniform development of large dark areas and the increase in latitude of development of continuous-tone images. In other wordsthey are less contrasty.

Grid 27 may be held at the same potential as plate 26 during development in which case the advantages already described are obtained. However, further control of development may be achieved by placing a potential on grid 27 which is different from the potential of backing plate 26. I I

Figure 5 shows a typical decay curve 59 for an electrophotographic plate. The ordinate of this graph represents the potential of the plate coating on a logarithmic scale after an exposure of a charged plate to the quantity of light indicated along the abscissa. For the curve shown the initial potential of the charge on the coating was in the order of 360 volts. It will be noted that as the quantity of light used for exposure of a given area increased, the potential decreased in substantially a straight line fashion on the semi-log scale to substantially 10 percent of the original potential after which the curve flattens out considerably. In making an exposure of a continuous-tone subject, it is preferred to regulate the exposure so that only the straight line portion of the curve is used, in order to achieve the best fidelity. If such an exposure is made it is evident that the exposure will be'stopped before the charge is completely removed from any area of the plate coating. Thus the most brightly illuminated area of the coating, corresponding to the high lights in the subject, may have-its potential reduced to the point 60 shown on the curve, while the middle tones will retain a higher potential 61 and the darkest areas of the image will retain nearly all of their original potential as indicated by the point 62.

It is apparent that if the latent image is now developed with powder that the charge remaining "on the high lights is capable of attracting some powder, although not as much as the charges on the middle tones and the dark areas. Thus with grid 27 at the same potential as the. backing plate, some development of the high lights will be obtained. This may be desirable for some purposes. In other cases, however, it will be desirable to prevent all deposition of powder on the high lights so that they will remain completely white, or nearly so, in the finished print. This can be accomplished by applying a potential to grid 27 substantially equal to the potential on the high-light areas as indicated by horizontal line 63. The preferred potential may most readily be determined by making one or more trial exposures under established conditions and developing the plate with tap 52 experimentally set at a predetermined potential. If the result of the first development indicates that some of the middle tones are not being developed, this means that the posi-' a enemies tiile; potential. thehgrid. should be 'lowered if fulien development of. the higher. middle tonesis. desired. It; however, sthe. high lights themselves tend to be: too dark, its-is. an indicationrthatthe potential applied to the grid was not high enough to get good high lights. Having once: established a grid. setting: for predetermined plate exposure, conditions, it is not thereafter' necessary to experiment .to obtain a the desired results.

The-potential control arrangement for grid. 27 also permitsi-adjustment of the'development for special purposes. Eoninstance, inzsonrecases it .maybedesired to develop only-the darkeareas of-.an imageand to; eliminate the middle;- tones" ass' well. as. the: high lights.- donebyraising-gthe electrode potential to ahigher value sucheasregual 'to thepotential s61so the; middle tones.

on,. me. otherhand, ifxa heavier powder deposit .is allrareas including;thehighlights. the poterr tiacl of gridlfl may actually .be-reversed. inpolarity. and made negative. with respect'to plate 26 ifsthe electrostatic-latent image is positiveI-andthe. developing powder ismegative. This'wiil intensity-the powder deposit over aihareasaof the-plate..

While itzis'preferred to establish the potential of .plate zfi byconnecting it to ground or. to aireference potential, it is not always essential that plate 26 beso connected during-tdeveloprnent. Thus if the plate 26 -is .substantially uncharged at therstart of'operation, the potentials applied to. grid..27 will have substantially'thesame effect on the fi'eid betweenthe field: and: the plate as if the plate were, actually, grounded or connected to a reference potential. v

. lids-3180 possible to improve: development of a latent image.- by.- cycling'; the potential :on the grid 1 during the development-as-was already indicated. Thus. it is possible. to compensate:- during development for. certain failuresof the-electrostatic image to correspond with the density; ofithe original being; reproduced.

Figured-is a graph of the potential of the electrostatic latent.-.image resulting frorna. certain exposure of an electrophotographic plate as '-a function of the optical density-*? the: original to which it was exposed. The curve 120:isnota straight line, since potentialdecay is nearly-astraight-line'fnnction with exposure rather. than optical :density; in: the original. (Density varies inversely as thelogarithmof the-exposure). Figure 6 indicates that aa given: ele'ctrophotographicplate which. has been chargedstora uniform potential and then given'a certain exposure ('asrdeterrnined :by intensity of illumination and time'fofiexposure) will have-vanielectrostatic:latent image on its surface in' which the potential will bGPl for an area- 0fthe'plate 'exposed' to.-a part of the-original having densityDn'and a density Pa for area exposed'to density Di" and=so"-forth Figure? shows the-substantially straight-line relationship betwe'enpotential on theelectrhphotographid plate andthe'optical densityfof'tl'ie developed final print over the potential. range of interest for various times of development, T1 to Ts, with development control grid 27'"set"at the high-light potential P0 (63 in Figure 5). Wi'tlishort development Ti, the density of all parts of the image. is. low, but is proportional to the potential differencebetween thepotentialon the image andflthe potential of naw. Progressively increased developmenttime's T2 to Te produce correspondingly increased densities in .each .part of the. image.

Figure .8. is-sa graphic :representation of adevelopment cycle whichswillsgivea densityv scale in.the'final print approximating th'edensity scale'of theoriginah The relationship-usually preferredfora the entirescale of densitiesiappearing=inzthe original canibe represented by straight dine'curve lzl. inlFigure 8, starting. at .the origin andkpassinga through: the series: :of. pointsin the. graph which represent densities 2i ofitheimagezequal.to2densities D in the original. Thus, for a point in the original This can. be

, .8, having merino. Di, the 'strai "litflinelourve lzlnindicatesa desired. density di=Di for theimage.

The curve 1'20in figure 6 can :be reasonably wellrep resented'hyia straight-line. up to a point corresponding to a'density .Dr andpjotentialPi. If-the valueofipotem tial Pnandthe value. of"d1('=Di) are marked onthe appropriate axes of Figure. 7these two valuesdetermine a coordinate point on this graph and this. point deter mines the proper development time, such as :Ta, required to achieve this relationshipofdensity and potential. At: the same time this onedensity area. is: being developed the. other density areas of the plate are also. being.

developed. These. areas will. generally correspond" to optical. densities in the original. frorn zero'up to. some. value between .one and-two; on the conventional. scale.

At. the end offlthis first development period, T s the density of the image:on theplatecanfberepresented by curve 122 iirli gima 8. 'It.-will be noted that theml'ower. densities,. from .zero. to di, oflthe image all icomereasonably close. to the ideal. densities determined by straight line 121. However, .thehigher densities are'-.insuifi.':iently developed -and-hence less dense thanthe higher-densitiesin the original It hasrbeentound that the higher density areascanbc broughtcloser to the-ideal curve 12 1 .bysuccessive peri* ods of development at highergrid potentials. Accord ing to. a preferred. method, aseeond period. of develop-- ment is added in: which the. potentialon the: develop-- ment grid 27 issraised-to potential-Prcorresponding to the density D1 "in the-original which is at the top of the" acceptably straightportion.. of curve in Figure 6. When thissis donetherewillrbe no substantial further dc velopment-insanyr areaeonthe plate which havepotentials lowerrthaniin These areasiare now completely devel oped. For the second period of the development-cycle;- the value Dz is selected sothat' curve 120'in Figure 6 and carve 1224a Figure: 8* are both reasonably well repre= sentedby straight .linesebetween the values- D1 andyDr'. Now-,1 to bring; the density; of the. print up-toithe desired straight line-in-Figure 8 'fordensity; Dz, it isnecessary'to add to :the" imageanincrement of density equalto'at; Figure '8. If the value of potential P2 and the value'ot" densityai are markedon the appropriate axes'of Figure 7 these values-determine .a coordinate point indicated by. the intersection: of. the? dashed. lines. Thispointdeterminesv the necessary: development time; suchl'as 14,011 a family of. straight line-curves 1 to moriginatingzat P1. These.curvesvareithesarne: as. curves-T1 torIs but are shifted to their'rght 'on thepotcntialaxis so that they orig inatezat B1 ratherthan-Pd. When thisseconddevelop mentperiod'-..is completed, the" final density dz' of the printarea-correspondingto the: original 'density of Dz1will fall on the'desiredlstraight'line: The: density of" the other areas .isindicatedby: curve 122.11-p to. density d1 an'dby curve-123ml? higherdensities;

By furtherraising the grid'potentialto equalPa'rand developing iorra furtherperiod determined by a similar process. thenh'igher: densities can. .be further. increased .as'. indicated by curve1-124. An. extension :of the PIOCBSSQ'OIH? lined above can: make: better and: better approximations to the straight line-desired between original .final density. From .three to five values. of. development-grid potential should besuificientforapractical purposes.

Theabove exposition of thezmethod of. arriving at the: proper. development cycle: can be: modified .in practice;- and it is 'POSSlblG'YXO; determine anacceptable develop.- ment.cycle-from:purely empirical tests. Inany case; a development cycle needs .to be :determined only once-.for' a particularicornbination of plate, totalexposure; anddevelopment operation. Actual iapplicationzof "the .methodi outhlnediabove indicates that practical values of develop:- ment timer-result- It mayrnot .alwayszbe desired to have: the :ir'na'gefequah the. densityyo'i the original ;and':it is evident that straight line curves can be drawn in Figure 8 which have greater or less slope than curve 121 and that development times can be correspondingly adjusted to produce image density ranges approximating these curve slopes.

Itis not always necessary that development at the various grid potentials shall take place in the sequence named. The sequence of development steps can be done in any order and it is even possible to cycle the grid potential through the sequence of values several times. The total time of development at each potential may be determined in the manner already indicated in connection with Figures 6 to 8 or by empirical tests. It is also possible to modify the process by varying the grid potential through a continuous series of values by sliding contact 52 up and down on the potentiometer (Figure 2) during development.

Figure 9 is a circuit diagram of a modified potential control circuit for the development grid. According to this arrangement an automatic cycling potentiometer 64 is arranged to vary the potential on grid 27 flirough a cycle of values during development. Thus the potentiometer may comprise a switch having a rotating contact 65 which moves step by step over five stationary contacts 66 which are connected together in series by four resistances 67, and the end contacts of the series being connected across the terminals of a battery 68. With a 200- volt battery and four equal resistances 67 of each of the contacts 66 will differ from its neighbor by 50 volts. Plate backing 26 of the electrophotographic plate may be connected to intermediate contact 66a so that the potential applied to moving contact 65 will be made both positive and negative with respect to the potential of plate 26 and also, when contact 65 reaches contact 66a, will be equal to the plate potential. The contact 65 may be connected directly to grid 27 but a further measure of control is afforded by connecting contact 65 by conductor 69 to the common terminal of the two l-volt batteries 70 and 71. The opposite terminals of the batteries are connected across a potentiometer 72 having a sliding tap 73 which is connected to control grid 27. It is thus possible to use tap 73 for manual cycling of the grid potential or to set tap 73 at any desired position to thereby adjust the potential applied by the automatic cycling potentiometer 64. It is obvious that plate backing 26 may be connected directly to one terminal of the battery 68, if desired; also that potentiometer 64 may be divided into as many sections as desired to reduce the voltage jump between sections as much as is wished. Any desired cycling, such as that described in connection with Figure 8, may be achieved by introducing the required voltage source and potentiometer sections. Moving contact 65 of potentiometer 64 is connected to a rotary shaft 74 which may be driven by an electric motor, such as motor 23, through a speed reducing de vice. With contact 65 driven at constant speed the time of development at each potential is determined by the relative lengths of stator contacts 66, these contacts being made of lengths proportional to the desired development times at each potential. According to a modification the position of contact 65is controlled by a program motor which is set to provide any predetermined cycling of the contact 65, and any predetermined length of dwell on each stator contact 66.

Figures and 11 illustrate an arrangement for developing an electrostatic latent image on a coating 35 of electrophotographic plate 26 with a finely-divided liquid spray or mist 131. A liquid 130 is placed in the reservoir 75 of air brush 76 and sprayed by compressed air feed through hose 77 into a space in which plate 26 is suspended face downward over a control, grid 78. Grid 78 may be similar to grid 27 described in connection with Figures 1 and 2. As illustrated the grid consists of an open-mesh screen of very fine wires supported between circular frame members 79. A spacer sheet 80 having a rectangular aperture is interposed between the top of the grid support frame and the coating 35 to p afiordthe desired spacingb'etween the coating.

Air brush may be of conventional construction to provide a liquid feed and'an air jet for atomizing the liquid and projecting it as a fine spray or mist. As shown in Figure 11 the working parts of the air brush comprise tubular elements surrounding a central pointed rod 83 providing a central cylindrical liquid passage 81 for feeding the liquid to be sprayed and a surrounding air passage 82 for converging the air around the liquid channel. Pointed rod 83 occupies the central axis of the liquid passage and terminates in a point at the outlet of the air brush. In order to apply a charge to the liquid particles as they are sprayed, a conductive ring electrode 84 is' supported around the tip of the air brush so that the jet of liquid droplets is projected out through the ring. Ring 84 is insulated from the air brush but is connected by an insulated conductor 85 to one terminal of a battery 86 or other potential source. The conductive parts of the air brush are connected to the other (grounded) terminal of the battery. This produces an electrostatic field between the liquid outlet of the air brush and ring 84 so that liquid particles leaving the brush acquire electrostatic charges by induction as they are ejected. Development of the electrostatic latent image is effected by blowing the liquid mist atmosphere 131 into the zone underneath the plate to be developed as shown in Figure 10.

In order to apply a charge to the liquid droplets as they emerge, it is preferable that a liquid which is at least slightly conductive be used. The following liquids have been successfully used with the apparatus shown in Figures 10 and 11 to develop continuous-tone electrophotographic images.

and the plate Example I The air brush reservoir was filled with India ink and the spray was directed toward the exposed plate held about two feet in front of the airbrush, while the ring 84 was held at a positive voltage of 1000 volts with respect to the airbrush. Plates were developed in this manner with the plate and grid assembly held in a vertical plane as well as in the horizontal plane shown in Figure 10. Best results were obtained with the plate horizontal and facing downward as shown in Figure 10 so that the surface of the plate is shielded from the direct spray. This allows freely floating droplets to become attracted to the plate without danger of larger droplets having a momentum imparted by the spray gun from being impinged directly against the plate surface. By changing the distance between the plate and airbrush the speed and uniformity of development can be varied. The airbrush was operated from an air pressure source of 30 lbs. per square inch and was adjusted to feed the ink at the maximum rate. Satisfactory development took place in most cases in 20'second's.-

Example 11 Water base inks such as fountain pen inks were sprayed in a similar manner and produced successful images. A 1% solution of water-soluble aniline blue, can be used, for example.

Y Example 111 A 10% solution of Oildag, an oil suspension of'colloidal graphite produced by Acheson Colloids Corp., Port Huron, Michigan, in carbontetrachloride produced a light image.

Example IV A 10% solution of printers ink in carbon tetrachloride produced a fine-grain image which could be transferred'to paper by pressure contact.

Example V 'Tneilnsl printwas transferred io-pa e'r and iutenstfied'by moistenin'g the paper slighty with alcohol.

' Example 'VI 7 transferring the lampblack image to paper electrostaticaily :andfa lso bypressing a layer of sheet material having a pressure-sensitive adhesive {such as rubber cement :or

moist gelatin) on its surface against the lampblack image. -It'i's apparent that the liquid spray development method permits a wide variety of inks and pigment materials to be used, permits ready control of the density of the cloud produced a-ndof the charge on the particles and .provides images which may berreadily transferred to paper or other surfaces.

Figure 12 illustrated a powder spray device including means-for imparting apredetermined charge on the powder particles. This comprises a flask 87 supported month downward by pedestal 88 from an insulating base 89. A stopper90 fitted in themouth of the flask is provided with three tubes. Inlet tube 91 fed through control valve 92 from :a source of compressed air terminates just inside the mouth of the :flask. A second tube 93 extends to the top of the inverted flask and is also connected through a control valve 94 with the compressed air source. The third tube 95 is an outlet tube and extends from near the top or the inverted flash out through the stopper and is bent at right angles so that the outlet end 96 will direct the issuing stream of air-suspended powder particles through asui'roundin'g metal ring or tube 97'supp'ortcdin horizonin position frombase 89. A fine wire Q8 is stretched alon the axis of tube '97 supported at one end by tube 19'5and the other "end by a terminal post 99. A high voltage direct current source is connected between terinitial 99 and tube 97 "to create a positive corona discharge from wire 98 to the inside walls of tube '97. in operation a charge of fine powder 100i's "placed in the-flask and valve 92 is opened 'to admit-compressed air through tube 9'1 to agitate the powder. valve 94 is also opened to admit compressediair through'ttibe 93to help Keep the pow'dertin suspension in the flask. Powder-laden arrzisdriven out through outlet tube 95 'and through'metal "tube 97 -where it acquires a positive charge from "the coronadiseharge. The issuing charged dust cloud may be used to develop images on plates supported over a control electrode, as shown in :Figure 10, for example. Suitable powders include finely-divided lampblack, wood charcoal, anthracite, pigmented resins such 'a'sthe Amber'ol-ibasc resin developer powder previously described and many other powdered materials.

While the present invention has general application to the development of electrostatic images on electropho- "t ra h'ie plates, el'ect'roprintin'g plates and the like, it is especially great value in the development of continuous-tone images "and of images containing large dark areas. In such cases the present invention has produced a marked improvement in qualityas well as enabled the development of relatively small vditferences in potential. l his has the effect ofinoreasin'gapparent speed of electrophotographic plates since a developa'ble image can be achieved by a much shorter exposure ofthe plate. For example, referring to Figure S, it is possible to give the plated; very short exposure so that the potentialon the high Flights-is reduced only slightly below the .initi'al po iteotia'l @to'which the plate was icha'rged. 'Thu's, ihcliigh- .-slig'ht:.potentiali=may be reduced only to ;point 'l'6l.iin-

stead of to point '60, and the middle tones and oa'r'ktone's swig-"lea will be proportionally higher. Upon development with a control grid set at a potential-equalto point 61 the potential differences in the reduced potential range are readily developed to yield a faithful reproduction of the otential differences in this reduced range.

It is also possible to obtain excellent development of plates initially charged to lower voltages than was possible heretofore. This is because of the effect of the.

. control electrode in increasing the electrostatic field 'in'the air outside the surface of the plate being developed. The lines of force which previously extended through the hi sulati'ng layer now extend out to the grid '27 so that they can influence powder or liquid particles floating or tumbling between the grid and the eleetrophoto graphic plate.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be lim ited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

.1. "The method of developing a continuous-tone electrostatic latent image on a surface, wherein said image comprises electrostatic charges of one polarity variously distributed on said surface to provide potentials which differ for difierent areas of said surface, the low potential areas corresponding to image high lights, the intermediate potential areas corresponding to middle tones and the potential areas corresponding to shadows and dark tones, which method compriseseffecting a pinrality of steps of partial development of said image by closely spacing a conductor in front of said surface and introducing an air suspension of a finely-divided material between said conductor and said surface, at least one of said steps being performed with said conductor held at high-light potential and other of said steps being performed with said conductorsheld at intermediate potenitials, said finely-divided material being charged to a polarity opposite to that of said image.

an xerographic method of developing an electrostatic image corresponding to a photographic reproduction of a pattern of light and shadow to be recorded, said image being .borne by a thin, uniform, photo-conductive insulating layer directly overlying a conductive surface, the developing method comprising positioning a conduc- It'ive. electrode substantiaIly coextensive with said image surface .andclosely spaced .firom said image surface at a distance. close enough to the electrostatic image so that the electrostatic lines of force that emanate outwardly from the different potential areas .on the electrostatic imagesurface tend to how mostly to the electrode rather than to-other .potent'ialareas, and projecting into the space Ibetweentheelectrode and the image surface agassuspen- ,sionof charged, finely-divided developer particles whereby .said developer .particles .are deposited improved conformity withthe electrostaticimage.

.3. Axerographic method ofdevelopingan electrostatic image corresponding to a photographic reproduction of .a .patternof light .andshadow to berecorded, said image being borne by a thin, uniform, .photocQnductive lusulating layer directly overlying a conductive surface, the :developing method. comprising positioning a conductive electrode substantially coextensive with said image ,surface-andcl'osely spaced from said image surface at a distance close enough to the electrostatic image. so that the. electrostatic lines of force that emanate outwardly "trons-the. dilfer'ent potential areas on the electrostatic ifrnag'esurfa'oejtend.toffibw mostly -to the electrode rather titanic/other potential areas, iandi'proiecting into the space between the electrode and the image surface a *gas suspension of charged, finely-divided liquid droplets of developer, whereby (said developer-is deposited proved conformity witht he 'o'lectrostaticimafge.

d. A'irero'graphic method-ot-developi g an electrostatic 13 image corresponding to a photographic reproduction of a pattern of light and shadow to be recorded, said image being borne by a thin, uniform, photo-conductive insulating layer, the developing method comprising positioning an insulating layer over said photoconductive insulating layer and in contact therewith and placing a conductive electrode substantially coextensive with said image surface and closely spaced from said insulating layer at a distance close enough to the electrostatic image so that the electrostatic lines of force that emanate outwardly from the different potential areas on the electrostatic image surface tend to flow mostly to the electrode through the said insulating layer rather than to other potential areas, and projecting into the space between the electrode and the insulating layer a gas suspension of charged, finely-divided developer particles, whereby said developer particles are deposited in improved conformity with the electrostatic image.

5. A Xerographic method of developing an electrostatic image corresponding to a photographic reproduction of a pattern of light and shadow to be recorded, said image being borne by a thin, uniform, photo-conductive insulating layer, the developing method comprising positioning an insulating layer over said photoconductive insulating layer and in contact therewith and placing a conductive electrode substantially coextensive with said image surface and at a distance close enough to the electorstatic image so that the electrostatic lines of force that emanate outwardly from the different potential areas on the electrostatic image surface tend to flow mostly to the electrode through the said insulating layer rather than to other potential areas, and projecting into the space between the electrode and the insulating layer a gas suspension of charged finely-divided liquid droplets of developer, whereby said developer is deposited in improved conformity with the electrostatic image.

6. The method of developing a continuous tone electrostatic image on a uniform photoconductive insulating surface wherein said image comprises electrostatic charges of one polarity variously distributed on said surface to provide potentials which differ for different areas of said surface, the low potential areas corresponding to image highlights, the intermediate potential areas corresponding to middle tones, and the high potential areas corresponding to shadows and dark tones, which method comprises closely spacing a conductor substantially coextensive with said image at a distance close enough to the electrostatic image so that the electrostatic lines of force that emanate outwardly from the different potential areas on the electrostatic image surface tend to flow mostly to the electrode rather than to other potential areas and introducing an air suspension of a finelydivided material between said conductor and said surface while maintaining said conductor at a potential corresponding to the highest potential on the said surface and of the same polarity as the potential on the said surface, said potential corresponding to the potential of that part of the surface representing the darkest tone of the image to be reproduced, said finely-divided material being charged to a polarity to that of the original image.

7. The method of developing a continuous tone electrostatic image on a uniform photoconductive insulating surface wherein said image comprises electrostatic charges of one polarity variously distributed on said surface to provide potentials which differ for different areas of said surface, the low potential areas corresponding to image highlights, the intermediate potential areas corresponding to middle tones, and the high potential areas corresponding to shadows and dark tones, which method comprises closely spacing a conductor substantially coextensive with said image at a distance close enough to the electrostatic image so that the electrostatic lines of force that emanate outwardly from the different potential areas on the electrostatic image surface tend to flow mostly to the electrode rather than to other potential areas and introducing an air suspension of a finely-divided material between said conductor and said surface while maintaining said conductor at a potential corresponding to the lowest potential on the said surface and of the same polarity as the potential on the said surface, said potential corresponding to the potential of that part of the surface representing the lightest tone of the image to be reproduced, said finelydivided material being charged to a polarity opposite to that of said image.

8. A xerographic method of developing an electrostatic image corresponding to a photographic reproduction of a pattern of light and shadow to be recorded, said image being borne by a thin, uniform, photoconductive insulating layer directly overlying a conductive surface, the developing method comprising positioning a conductive electrode in the form of a grid substantially coextensive with said image surface and closely spaced from said image surface at a distance close enough to the electrostatic image so that the electrostatic lines of force that emanate outwardly from the different potential areas on the electrostatic image surface tend to flow mostly to the electrode rather than to other potential areas, and projecting into the space between the electrode and the image surface a gas suspension of charged, finely-divided developer particles whereby said developer particles are deposited in improved conformity with the electrostatic image.

9. The method of making a visible pattern on a surface which comprises forming an electrical charge-pattern on said surface in which different areas have different potentials, positioning an electrically-conductive foraminous grid structure so that it is separated from but disposed close enough to said surface so that the electrical lines of force that emanate outwardly from the different potential areas on said surface tend to flow mostly to the grid structure rather than to other potential areas on said surface,

and directly previously electrically-charged marking par-' ticles through said grid structure into the electrical field of said pattern.

References Cited in the file of this patent UNITED STATES PATENTS 1,784,912 Scott Dec. 16, 1930 1,958,406 Darrah May 15, 1934 2,152,077 Meston et al Mar. 28, 1939 2,173,032 Wintermute Sept. 12, 1939 2,191,827 Benner et a1. Feb. 27, 1940 2,281,638 Sukumlyn May 5, 1942 2,302,289 Bramston-Cook' Nov. 17, 1942 2,421,787 Helmuth June 10, 1947 2,425,652 Starkey Aug. 12, 1947 2,476,145 Gwyn et al. July 12, 1949 2,551,582 Carlson May 8, 1951 2,573,881 Walkup et al Nov. 6, 1951 2,600,121 McGee et a1. June 10, 1952 2,633,796 Pethick Apr. 7, 1953 2,691,343 Huebner Oct. 12, 1954 2,691,345 Huebner Oct. 12, 1954 FOREIGN PATENTS 605,979 Great Britain Aug. 4, 1948 

1. THE METHOD OF DEVELOPING A CONTINUOUS-TONE ELECTROSTATIC LATENT IMAGE ON A SURFACE, WHEREIN SAID IMAGE COMPRISES ELECTROSTATIC CHARGES OF ONE POLARITY VARIOUSLY DISTRIBUTED ON SAID SURFACE TO PROVIDE POTENTIALS WHICH DIFFER FOR DIFFERENT AREAS OF SAID SURFACE, THE LOW POTENTIAL AREAS CORRESPONDING TO IMAGE HIGH LIGHTS, THE INTERMEDIATE POTENTIAL AREAS CORRESPONDING TO MIDDLE TONES AND THE HIGH POTENTIAL AREAS CORRESPONDING TO SHADOWS AND DARK TONES, WHICH METHOD COMRISES EFFECTING A PLURALITY OF STEPS OF PARTIAL DEVELOPMENT OF SAID IMAGE BY CLOSELY SPACING A CONDUCTOR IN FROMT OF SAID SURFACE AND INTRODUCING AN AIR SUSPENSION OF A FINELY-DIVIDED MATERIAL BETWEEN SAID CONDUCTOR AND SAID SURFACE, AT LEAST ONE OF SAID STEPS BEING PERFORMED WITH SAID CONDUCTOR HELD AT HIGH-LIGHT POTENTIAL AND OTHER OF SAID STEPS BEING PERFORMED WITH SAID CONDUCTOR HELD AT INTERMEDIATE POTENTIALS, SAID FINELY-DIVIDED MATERIAL BEING CHARGED TO A POLARITY OPPOSITE TO THAT OF SAID IMAGE. 